Re: [opendiag] So is the VAG protocol preferrable to ISO-9141-2? (more)

On Fri, Jul 28, 2000 at 05:51:53PM -0000, blg@... wrote:
> You guys mentioned earlier that the VAG protocol allowed access to
> more data than the ISO-9141-2? Is this true? Does this mean that
> the
> ISO-9141-2 standard isn't fully implemented and supported in these
> cars?

ISO9141-2 is a method for transmitting and receiving bits on a wire.
It also specifies the framing of the data. It doesn't specify what the
data itself is used for -- that's what OBDII is for. OBDII doesn't care
about the low level protocol.

The VAG protocol, as I understand it, is a variation of ISO9141-2, *and*
a superset of OBDII. It supports all the OBDII functionality as well
as vendor-specific enhancements, such as the ability to program the
central locking system. It has the advantage of running at a standard
bit rate.

It may be possible to use the VAG extensions to OBDII, while at the same
time using the ISO9141-2 layer to communicate with the ECU. As far as
I know, it has not been established whether or not this is possible.
I personally do not care, because the VAG layer is much simpler to
interface with.

I am not aware of any source of documentation for the VAG communications
layer or protocol. It is possible that documentation can be obtained
from Bosch and/or SAE. (VWtool's author had to get it from somewhere
... or perhaps he reverse engineered a VAG tester! Sadly we will never
know.)

Garrett Lim once told me he was able to obtain detailed specs on the Bosch
electronics. I know SAE sells lots of Bosch manuals. Perhaps it's in one
of those. (He is able to flash his performance software into ECUs without
so much as popping the hood. He doesn't do it because he fears piracy.)

If you find any documents on the VAG protocols, I would love to know
about it. In the meantime, it should be relatively easy to reverse
engineer parts of it from VWtool, VAG-COM, and the OBDII specs.

The VAG protocol works on cars as early as 1991, perhaps earlier.
OBDII became the standard for 1995 model year cars. Baumtools.com has
more information on which cars support this protocol.

You can rest assured, however, that ISO9141-2 and OBDII are fully
implemented in these cars (at least 1995-2000 model years.) OBDII
compliance is federally mandated on cars sold in the USA.

To be honest, I think you've exhausted the knowledge of everyone on this
mailing list. :-)

Regards,

Jeff

 
Received on Fri Jul 28 11:11:20 2000